This invention relates to load pull testing of radio frequency transistors and amplifiers under high power operating conditions, using automatic impedance tuners in order to synthesize impedances at the input and output of the test devices (DUT) at the fundamental and various harmonic frequencies.
Accurate design of high power amplifiers, oscillators and other active components used in various communication systems requires accurate knowledge of the active device's (RF transistor's) characteristics under high power operation conditions. In such circuits, it is insufficient and inaccurate to describe transistors, operating at high power in their highly non-linear regions close to saturation, using analytical or numerical models only. Instead the transistors need to be characterized using specialized test setups under the actual operating conditions.
A popular method for testing and characterizing such components (transistors) under high power operation conditions is “load pull” and “source pull”. Load pull or source pull are measurement techniques employing RF impedance tuners and other RF test equipment, such as RF signal sources and RF power meters. Other components include frequency discriminators (Diplexers or Triplexers), whose task is to separate the harmonic components generated by the DUT into different paths, where they can be treated separately (FIG. 1).
The impedance tuners are used in order to manipulate the microwave impedances presented to and under which the Device under Test (DUT, amplifier or transistor) is tested (FIG. 1).
There are essentially three types of tuners used in such test setups: a) Electro-mechanical slide screw tuners [1], b) Electronic tuners [2] and c) Active tuners [3].
Electro-mechanical slide screw tuners [1] have several advantages compared to electronic and active tuners, such as long-term stability, higher handling of microwave power, easier operation and lower cost. In this type of tuner a dielectric or metallic semi-cylindrically bottomed RF probe (slug) is inserted into the slot of a slotted transmission airline as described before [4], FIG. 2; this insertion of the slug allows reflecting part of the power coming out of the DUT and creating a complex reflection factor (Γ) or complex impedance (Z) that is presented to the DUT.
The known relation between Γ and Z is: Z=Zo*(1+Γ)/(1−Γ); where Zo is the characteristic impedance of the transmission line (slabline) in which the slug is inserted. A Zo value of 50Ω or 75Ω is typically used in this frequency range.
There are two major obstacles for making such electro-mechanical slide screw tuners working at radio frequencies below 100 MHz: One is the limited achievable capacitance between the RF probe and the central conductor (FIG. 2) and the other is the required length of the transmission line of the tuner.
Electromechanical slide screw tuners need to be at least one half of a wavelength (λ/2) long, in order to be able to create a phase rotation by at least 360° in order to synthesize reflection factors Γ covering the entire Smith Chart [4]. At a frequency of 1 GHz this corresponds to a length of 15 centimeters, at 100 MHz this becomes 1.5 meters and at 10 MHz it becomes 15 meters. It is obvious that it is not possible to manufacture affordable and precise slotted airlines (slablines) bigger than 1.5 to 2 meters and use them in a RF laboratory environment. Or the minimum realistic frequency for conventional slide screw tuners is 100 MHz.
Therefore a new structure for automatic tuners has been used, the triple capacitor structure [5]. In this apparatus three mechanically adjustable air capacitors are used, which are separated by lengths of coaxial RF cable. This compact configuration allows tuning over a wide range of reflection factors, FIGS. 3, 15. This tuner uses fixed lengths of semi-rigid cable in coil form in series between shunt capacitors and has limited frequency bandwidth.
Because the three capacitor tuner [5] uses fixed lengths of transmission line between capacitor sections it allows impedance tuning over a full coverage of the Smith chart only at over a narrow frequency range. Assuming 10 settings of each capacitor, between minimum value and maximum value, the total number of states of the three cascaded tuning sections will be 10*10*10=1000. Because the length of cables between capacitors is fixed the phase of the individual reflection factors created by each capacitor cannot be rotated. This restricts the tuning range to the frequencies for which the transmission line lengths have been optimized [5].
A new tuner structure is used here which uses variable lengths of cable between the variable capacitors, in form of low frequency phase shifters; this apparatus is capable of wideband tuning and harmonic tuning.